Vehicle fuel systems include evaporative emission control systems designed to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel. For hybrid vehicles, other vehicles with limited engine run time, or other vehicles configured to operate with minimal intake manifold vacuum, opportunities to purge a fuel vapor canister to intake may be limited. In Non-Integrated refueling canister only systems (NIRCOS), the fuel tank is typically sealed via a closed FTIV except during refueling operations.
However, the fuel tank may periodically be subject to significant pressure differentials, either positive pressure or vacuum. In order to maintain the integrity of the fuel tank and fuel system, as well as to allow for predictable fuel tank depressurization times prior to refueling, the fuel tank may be instructed to vent excess pressure from fuel vapor. While the vented fuel vapor may be adsorbed by the fuel vapor canister, at extreme pressures, opening the fuel tank isolation valve may overwhelm the canister, causing fuel vapor to be released. Typically, such a fuel tank venting event is performed while the engine is running, so that unbound fuel vapor can be directed to the engine intake for combustion, and/or the fuel vapor canister can be purged of its contents. However, if the engine is not running, fuel vapor may be released as evaporative emissions during a fuel tank venting event.
Other attempts to address fuel tank depressurization include providing both a refueling valve and a tank pressure control valve between the fuel tank and fuel vapor canister. One example approach is shown by Peters et al. in U.S. Patent Application 2015/0068498. Therein, a tank pressure control valve is provided with a smaller orifice than for the refueling valve, and is used to reduce fuel tank pressure during engine-on operations.
However, the inventors herein have recognized potential issues with such systems. As one example, for hybrid vehicles, venting the fuel tank in this manner cannot be performed during engine-off vehicle operation, as undesired emissions may occur. Further, the use of multiple valves and conduits adds system cost and complexity, requiring additional diagnostic tests.
In one example, the issues described above may be addressed by a method for a fuel system. During a first condition that includes a fuel tank pressure above a threshold, a fuel tank is fluidically coupled to a fuel vapor canister while a fuel vapor canister vent path is restricted, and a degree of restriction of the fuel vapor canister vent path is modulated. Modulating the degree of restriction of the fuel vapor canister vent path allows a fuel tank to be depressurized at a controlled rate. In this way, excess fuel tank pressure can be relieved, even if a vehicle engine is not operating in a combustion mode.
As one example, modulating the degree of restriction of the fuel vapor canister vent path may include adjusting a position of a changeover valve within an evaporative leak check module coupled within the fuel vapor canister vent path. In this way, existing components can be used to depressurize the fuel tank in a controlled fashion, and a canister purge valve may be maintained closed during fuel tank venting, thereby enabling engine-off fuel tank depressurization.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.